Signalling

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Railway
signalling is a complex and fascinating subject. This site has a number
of pages explaining the signalling and train control systems in use
around the world ranging from old semaphore signals still used in the UK
and elsewhere to modern electronic high capacity systems used by
metros. We also provide links to other railway signalling sites around the world that describe local systems.

Figure 1: Colour light signals on a local railway in Japan. Photo: PekePON.

Background

Signalling
is one of the most important components of the many which make up a
railway system. Train movement safety depends on it and the control and
management of trains depends on them. Over the years many signalling and
train control systems have been evolved so that today a highly
technical and complex industry has developed. Here is an attempt to
explain, in simple terms, how railway signalling developed and how it
really works, based on the UK standards.

Pioneer Signalling

Back
in the 1830s and 40s (in the very early days of railways) there was no
fixed signalling - no system for informing the driver of the state of
the line ahead. Trains were driven "on sight". Drivers had to keep their
eyes open for any sign of a train in front so they could stop before
hitting it. Very soon though, practical experience proved that this
didn’t work and there needed to be a way of preventing trains running
into each other. Several unpleasant accidents had shown that there was
much difficulty in stopping a train within the driver's sighting
distance. The problems were, partly, inexperience and poor brakes but
the real problem was (and still is) the rather tenuous contact which
exists on the railway between steel wheel and steel rail for traction
and braking. The adhesion levels are much lower and vehicle weights much
higher on railways than on roads and therefore trains need a much
greater distance in which to stop than, say, an automobile travelling at
the same speed. Even under the best conditions, it was (and is even
more so nowadays with high speeds) often impossible to stop a train
within the sighting distance of its driver.

The Time Interval System

In
the early days of railways, it was thought that the easiest way to
increase the train driver's stopping distance was to impose time
intervals between trains. Most railways chose something like 10 minutes
as a time interval. They only allowed a train to run at full speed 10
minutes after the previous one had left. They ran their trains at a 10
minute "headway" as it is called.

Red,
yellow and green flags were used by "policemen" to show drivers how to
proceed. A red flag was shown for the first five minutes after a train
had departed. If a train arrived after 5 minutes, a yellow caution
signal was shown to the driver. The full-speed green signal was only
shown after the full 10 minutes had elapsed.

The
"time interval system", in trying to use a headway to protect trains,
actually created some serious problems of its own. The most serious was
that it was still inherently dangerous. Trains in those days were
considerably less reliable than they are today and often broke down
between stations. It also could not be guaranteed that the speed of the
first train would be sufficient to prevent the second catching it up.
The result was a series of nasty rear-end collisions caused, in each
case, because the driver believed he had a 10 minute gap ahead of him
and had little or no warning if there was an erosion of that 10 minutes.
Even if the time was reduced so much that he could see the train in
front, he often did not have enough braking capacity to avoid a
collision.

Line Capacity

Another
serious problem, from the railways' point of view, was line capacity.
Even if they could rely on their trains not to make unscheduled stops
and all to travel at the same speed, the 10 minute time interval
restricted the number of trains which could run per hour (in this case
6) over a given line. As they found they needed to run more trains, they
gradually began to reduce the time between trains. As they reduced the
time, or "headway", the number of trains per hour increased. At the same
time too, the number of accidents increased. Eventually, they realised
they had to do something. The answer was fixed signalling.

Fixed Signalling

Even
with the old time interval system, the basic rule was to divide the
track into sections and ensure that only one train was allowed in one
section at one time. This rule is still good today. Each section (or
block as it is often called) is protected by a fixed signal placed at
its entrance for display to the driver of an approaching train. If the
section is clear, e.g. there is no train in it, the signal will show a
"Proceed" indication. For many years in Britain it was usually a raised
or lowered semaphore arm (Figure 2). There are a few of these left
around the country but nowadays it is usually a green light or "aspect",
as the railways call it. If, however, the section is occupied by a
train, the signal will show a "Stop" indication, usually a red aspect.
The next train will be made to wait until the train in front has cleared
the section. This is the basis upon which all signalling systems are
designed and operated.

Figure
2: Photo of typical railway semaphore signals at the Great Western
Society site at Didcot, Oxfordshire, UK. The signal on the right
displays a stop command, while the one on the left displays
'proceed’ for the diverging route on the left. Photo: Author.

Mechanical
signals first appeared in the UK in 1841 and a signal box with levers
controlling remote signals and points in 1860. Originally, the passage
of each train through a section was tracked visually by the signalman.
When the train had cleared his section, the signalman told the signal
box on the approach side that his section was now clear and that he
could, if required, "accept" another train. The messages between signal
boxes were transmitted by a system of bell codes using the electric
telegraph.

Compulsory
use of the electric "block telegraph" to pass messages, and signal
interlocking, where points and signals were mechanically prevented from
allowing conflicting movements to be set up, were introduced in the UK
following the Regulation of Railways Act of 1889.

Distant Signals

The
basic stop/go signal used to protect each section of the line was OK as
long as the driver of an approaching train was able to see the signal
in time to stop. This was rarely the case, so a system of "distant"
signals was provided in many locations.

Distant
signals were placed in such a position that the driver could stop in
time if the next stop signal was at danger. Positioning depended on the
visibility, curvature, maximum permitted line speed and a calculation
of the train's ability to stop. In the UK, freight trains with reduced
braking capacity (unfitted or partially fitted freights) were only
allowed to run at restricted top speeds to allow for signal braking
distances.

Originally,
distant signals were semaphores, like the stop signals mentioned
above. They showed a green light at night if their related stop signal
was also green (or clear) and yellow if the stop signal was at red. The
red-yellow-green pattern was adopted for colour light signals and
eventually used to provide a more spohisticated form of train control.

Interlocking

Another
safety feature introduced in the mid-19th Century was mechanical
interlocking of points and signals. The purpose was to prevent the route
for a train being set up and its protecting signal cleared if there was
already another, conflicting route set up and the protecting signal for
that route cleared. The interlocking was performed by a series of
mechanically interacting rods connected to the signal operating levers
in the signal box. The arrangement of the rods physically prevented
conflicting moves being set up. As the systems developed, some larger
signal cabins at complex junctions had huge frames of interlocking
levers, which gave the name "lever frame" to the row of operating levers
in a signal box.

Eventually,
by the time signal levers were being replaced by small (miniature)
levers or push buttons, mechanical interlocking frames were superseded
by relay interlockings. Electro-magnetic relays were used in series to
ensure the safety of route setting at junctions. Complex "control
tables" were drawn up to design the way in which these relays would
interact and to ensure safety and integrity. Now, most of this
is computerised.

Blocks

Figure
3: Schematic of signal block section. When a block is unoccupied, the
signal protecting it will show green. If a block is occupied, the signal
protecting it will show red. Diagram: Author.

Railways
are provided with signalling primarily to ensure that there is always
enough space between trains to allow a following train to stop before it
hits the one in front. This is achieved by dividing each track into
sections or "blocks". Each block is protected by a signal placed at its
entrance. If the block is occupied by a train, the signal will display a
red "aspect" as we call it, to tell the train to stop. If the section
is clear, the signal can show a green or "proceed" aspect.

The
simplified diagram (Figure 3) shows the basic principle of the block.
The block occupied by Train 1 is protected by the red signal behind it
at the entrance to the block. The block behind (“in rear”, as it is
known) is clear of trains and a green signal will allow Train 2 to enter
this block. This enforces the basic rule or railway signalling that
says only one train is allowed onto one block at any one time.

The Track Circuit

Nowadays
for signalling purposes, trains are monitored automatically by means of
"track circuits". Track circuits were first tried in the US in the
1890s and soon afterwards appeared in Britain. London Underground was
the first large-scale user of them when they introduced them in 1904-6
as part of their electrification programme.

Low
voltage currents applied to the rails cause the signal, via a series of
relays (originally) or electronics (more recently) to show a "proceed"
aspect. The current flow will be interrupted by the presence of the
wheels of a train. Such interruption will cause the signal protecting
that section to show a "stop" command. Any other cause of current
interruption will also cause a "stop" signal to show. Such a system
means that a failure gives a red aspect - a stop signal. The system is
sometimes referred to as "fail safe" or "vital". A "proceed" signal will
only be displayed if the current does flow. Most European main lines
with moderate or heavy traffic flows are equipped with colour light
signals operated automatically or semi-automatically using track circuit
train detection.

Track Circuit - Block Unoccupied

Figure
4: This diagram (right) shows how the track circuit is applied to a
section or block of track. A low voltage from a battery is applied to
one of the running rails in the block and returned via the other. A
relay at the entrance to the section detects the voltage and energises
to connect a separate supply to the green lamp of the signal. Diagram:
Author.

Track Circuit - Block Occupied

Figure
5: When a train enters the block (right), the leading wheelset short
circuits the current, which causes the relay to de-energise and drop the
contact so that the signal lamp supply circuit now activates the red
signal lamp. The system is "fail-safe", or "vital" as it is sometimes
called, because any break in the circuit will cause a danger signal to
be displayed. Diagram: Author.

The
above provides a simplified description of the track circuit. The
reality is somewhat more complex. A block section is normally separated
electrically from its neighbouring sections by insulated joints in the
rails. However, more recent installations use electronics to allow
jointless track circuits. Also, some areas have additional circuits
which allow the signals to be manually held at red from a signal box or
control centre, even if the section is clear. These are known as
semi-automatic signals. Even more complexity is required at junctions.

Multi-Aspect Signals

The
basic, two-aspect, red/green signal is fine for lower speed operation
but for anything over about 50 km/h the driver of a train needs a
warning of a red signal ahead to give him room to stop. In the UK, this
led to the idea of caution signals (originally called "distant" signals
when they were mechanically operated semaphore arms) placed far enough
back from the signal protecting the entrance to the block to give the
driver a warning and a safe braking distance in which to stop. When
this was developed for track circuited signalling, the caution signal
was provided a block further back from the stop signal. Each signal
would now show a red, yellow or green aspect - a multi-aspect signal.

The
diagram (Figure 6) shows a line with 3-aspect signals. The block
occupied by Train 1 is protected by the red signal at the entrance to
the block. The block behind is clear of trains but a yellow signal
provides advanced warning of the red aspect ahead.This block provides
the safe braking distance for Train 2. The next block in rear is also
clear of trains and shows a green signal. The driver of Train 2 sees the
green signal and knows he has at least two clear blocks ahead of him
and can maintain the maximum allowed speed over this line until he sees
the yellow.

Four-Aspect Signalling

The
multi-aspect signalling commonly used in the UK today is a 4-aspect
system. It works similarly to the 3-aspect system except that two
warnings are provided before a red signal, a double yellow and a single
yellow. This has two purposes. First, it provides early warnings of a
red signal for higher speed trains or it can allow better track
occupancy by shortening the length of the blocks. The high speed trains
have advanced warning of red signals while the slower speed trains can
run closer together at 50 km/h or so under "double yellows".

Figure 7: Schematic (right) of 4-aspect signalled route showing how the double-yellow aspect works. The
upper diagram shows four-aspect signals with a high speed train with
three clear blocks ahead of it and then, in the lower diagram, a slower
train with two clear blocks ahead of it. The lower speed trains can run
closer together so more trains can be operated over a given section of
line. Daigram: Author.

A Safe Braking Distance

The
foregoing description of signalling has so far only looked at the
concept of warning or enforcement of restrictive signal indications. It
has not yet taken into account braking distance or headway. First, there
is the problem of braking distances. As we have already seen, a train
cannot stop dead. An Inter City train travelling at 100 mph (160 km/hr)
will take more than a mile to stop. Even for a signalling system with
enforcement (ATP) like the London Underground, as described so far there
is a risk that a train could pass a stop signal, then be stopped by the
ATP enforcement system and still hit the train in front. This
situation could occur if the train in front was standing just ahead of
the signal protecting it. The problem has long been recognised and can
be overcome by the provision of a space for the train to stop in, an
"overlap".

The Overlap

In
its simplest form, the overlap is a distance allowed for the train to
stop in should it pass a signal showing a stop aspect. It is provided by
positioning the signal some way before the entrance to the section it
is protecting.

Figure
8: Photo of typical British railway 4-aspect colour light signal
showing a double yellow aspect. The bottom lens is for the red aspect
and the one between the two yellows is for the green. Photo: WBSS Co.

On
railway in Britain, because it is impossible to calculate all the
various braking distances of different types of trains and because it is
impossible to predict when a driver might react to a stop signal, a
fixed value of 200 yards (185 metres) is used. On metros that use ATP
systems, the distance is calculated by a precise formula based on the
known braking capacity of the metro train, the gradient at the location
concerned, the maximum possible speed of the trains using that section,
an allowance for the sighting of the signal by the driver and a small
margin. The result of the calculation is called the "safe braking
distance". The overlap incorporates this safe braking distance.